The invention relates to an electromechanical apparatus for conversion of force to electric signal, comprising a part receiving the force to be measured, measuring element and a part transferring the reaction force.
It is known that the mechanisms presently in technical use are based on the equilibrium of the structural force-effects. Measuring of these force effects is a frequent task in the technical life. Such tasks are associated with the further demands that in addition of the force to be measured, the dynamic forces (e.g. vibrations) arising from the environmental effects should also be eliminated during the measuring process.
The measuring tasks are frequently carried out with electric measuring method. The force measurement with electric method appeared and was generally used in the industrial measuring technique some decades ago. Conversion of the force and electric signal is accomplished primarily with the help of the so called piezo-resistivity principle. In such measuring converters the piezo-resistive measuring sensor is generally arranged on the measuring element. The measuring element suffers deformation between two mechanical poles, i.e. the active force and reaction force. Accordingly, the equilibrium of the forces is brought about through the mechanical stress field developing in the measuring element. The measuring sensing resistances on the surface of the measuring element will be altered as a result of deformation taking place under the influence of the mechanical stress, which generally upsets the equilibrium of an electric measuring bridge.
The dynamometer cells, as measuring converters can be described generally possess characteristics which depend on the formation of the measuring sensor, measuring element and measuring bridge. Such measuring converters are frequently used for instance in the industrial scale-systems, or in the measuring apparatuses of the electronic material testing machines.
For correct operation, the suitable incorporation of the measuring converters is necessary into the mechanical systems. The fundamental condition for this is that only the force to be measured should participate in the force-electric signal converter. For this reason it is essential that the measuring direction of the measuring converter and the influence line of the force to be measured should coincide, i.e. the mutual position should not be altered during the measuring process. Similarly important is that the properly fixed point of application of the force and displacement of this point of application during deformation as taking place under the influence of the force should occur without the development of additional forces and additional stress fields.
Several constructions have been developed in order to fulfill simultaneously the above described conditions for correct operation. Known is, for instance, such solution, where the force is transferred with the aid of intermediate elements. In this case the point of application of the force is on the intermediate element. Such intermediate element may be, for instance, a ball arranged in a nest, or wedge fitting into a trough or slot. A lentiform or flexible washer fitting to the end of a shaft is also applicable.
The point of application of a force may be directed also on the measuring element. In this case, it is a frequent solution that the surfaces of the measuring element carrying the point of application of the force or reaction force are spherical surfaces.
There are also known constructions where the reaction force arises on a field consisting of balls.
However none of these solutions ensure the complete fulfilment of the earlier mentioned conditions.